(1) Field of the Invention
This invention relates to a solid-liquid separator for use in a primary treatment of waste water containing solid matter, and to a process for washing the solid-liquid separator.
(2) Description of the Prior Art
A method of washing a downward flow type filter according to the prior art, based mainly on sand filtration, is shown in FIG. 5. The system shown in the figure comprises a tank 1 packed with a layer of sand 2, a washing water tank 3, a washing pump 4, a surface washing water pipe 5, a backwash pipe 6, control valves 7 to 9, and a backwash drain pipe 10.
In washing the filter, the washing pump 4 is operated and the control valves 7 to 9 are controlled, whereby washing water is first pumped up from the washing water tank 3 and is sprinkled through the surface washing water pipe 5 to wash the surface of the sand layer 2 (surface washing). Then, the washing water in the tank 3 is fed under high pressure through the back washing pipe 6, which serves also as a treated water pipe, into a lower portion of the sand layer 2, thereby washing the entire part of the sand layer 2 (back washing). In this step, the sand layer 2 is expanded, and the suspended solids (SS) caught in space portions in the sand layer 2 are removed from the space portions, to be discharged through the backwash drain pipe 10. In some cases, air washing is also used, prior to the washing with water.
The conventional method of washing a sand filter requires huge expenses for operation and maintenance, because the method uses a large amount of washing water for surface washing and back washing. In addition, the method requires additional equipment such as the washing water tank, washing pump, etc., leading to a high equipment cost as well as a problem on a spatial basis.
Meanwhile, in the sand filter, the capture of the suspended solids (SS) takes place concentratedly in the vicinity of the surface of the sand layer, and the SS are practically caught in voids between the grains of sand. Therefore, the above-mentioned washing with water is enough to remove the SS from the sand filter.
Referring to FIG. 4, there is shown a filter medium 11 which is used in a solid-liquid separator according to the present assignee's prior application [for instance, Japanese Patent Application Laid-Open (KOKAI) No. 63-32861 (1988)]. In use of such a filter medium, namely, a small cylindrical plastic filter medium having a specified gravity of not more than 1.0 and a void ratio of at least 70%, suspended solids are not only caught in the voids between the pieces of filter medium 11 but are caught, in considerable amounts, in space portions 11a constituting the interior of the cylindical pieces of filter medium 11. Besides, the SS are not only caught on the surface of the packed bed but are caught in a high density throughout the range of the packed bed (for instance, 1.5 to 2 m deep). In this case, therefore, it is impossible to achieve satisfactory removal of the SS from the space portions 11a of the filter medium 11 by the above-mentioned water washing.